1. Field of the Invention
This invention relates to spread-spectrum communication and, more particularly, to a communication system using multiple communication modes over multiple frequency bands.
2. Description of Related Art
Cellular telephony has been well known for many years, but with its growing popularity, more channels in the allocated cellular frequencies have become necessary. Among the proposed advances in the art have been a move from frequency division multiple access (FDMA) systems using narrowband analog communication to digital voice communication using traditional narrowband FDMA techniques possibly coupled with time division multiple access (TDMA) techniques. Further proposed advances include the use of code division multiple access (CDMA) techniques such as spread spectrum systems. Examples of communication protocols include IS-45, IS-95, DCS1900 (otherwise known as GSM), DECT (Digital European Cordless Telephone), and AMPS.
Another approach to the problem of allowing increased numbers of users in a geographic location is the concept of personal communications systems, or PCN's, which utilize microcells. A microcell is similar to a cell in a traditional cellular system, except much smaller. Where a traditional cell may cover an area of several square miles, a microcell may only be a few hundred feet in diameter. By limiting transmit power, more microcells, and thus more users, may be co-located in a geographic area.
Prior art does not teach a method for operation of a single telephone which has the ability to function both as a narrowband frequency, time, and/or code division multiplexed cellular phone, as well as a microcellular telephone utilizing time, frequency, or code division multiplexing, where the cellular and microcellular functions either share the same frequency bands of operation or are offset from each other. Nor does the prior art teach such a system where the microcellular mode may employ a paging unit independent of the unit's telephone functionality.
For purposes of the present specification, "analog voice" is described as a system where an analog voice system directly modulates a radio frequency (RF) carrier or intermediate frequency (IF) signal, and digital voice is described as a system where the signal is first digitized, and possibly compressed through any number of methods common and well known in the art, and whose digital signal is then used for RF carrier or IF modulation. A narrow band modulation typically uses amplitude modulation (AM) or frequency modulation (FM), and has a bandwidth between 3 kHz and 30 kHz.
In spread-spectrum communication, the spread-spectrum signal which is generated and transmitted has a spreading bandwidth which exceeds the bandwidth of the data stream. When using spread-spectrum techniques for wireless communication, it may be necessary to avoid or minimize interference with other users of the electromagnetic spectrum. Some examples of such other users are microwave communication users (such as the Operational Fixed Services ("OFS") using microwave communication towers) and cellular communication users (such as those using cellular telephones). In particular, OFS services are critical to controlling, among other things, the nation's electric power grid, which makes the possibility of inadvertent OFS disruption extremely serious. Accordingly, it would be advantageous to avoid or minimize interference with microwave and cellular communication users.
In wireless communication, the transmitted signal may be subject to various forms of frequency-selective fading, which may cause the signal to fade or drop out over a localized range of frequencies. Although spread-spectrum signals are distributed over a wider range of frequencies than narrowband signals, they may also be subject to frequency-selective fading over a portion of their spreading bandwidth. Accordingly, it would be advantageous to mitigate the effect of frequency-selective fading.
Spread-spectrum modulation in more than one frequency band can be difficult due to the wide separation between frequency bands. For example, operation in the 900 megahertz and 1800 megahertz bands could require a synthesizer capable of covering approximately 1,000 megahertz in frequency spectrum. However, in hand-held equipment such as telephones, it is undesirable to use more than one synthesizer, or even more than one oscillator, due to increased cost, weight, and related considerations. Accordingly, it would be advantageous to provide a spread-spectrum system in which a single, relatively narrow, synthesizer would serve more than one operating frequency band.